1. Field of the Invention
The invention relates to a process and a device for mixing and piping suspensions of different natures and/or compositions in the stable section of a paper machine.
2. Description of Background Information
A process and device for mixing suspensions is known from U.S. Pat. No. 4,477,313 to Andersson, issued Oct. 16, 1984. According to the Andersson patent, the backwater collected in the paper machine is passed into open backwater tanks, and is then fed back to the headbox via mixing pumps provided with a thick stock supply.
If a change in paper type is made on the paper machine, drainage conditions usually change and, thus, the concentration (e.g., solid content) of the backwater (in particular) usually changes. However, in the Andersson patent, due to the high residence time of the backwater in the backwater tank, the concentration in the backwater tank changes only slowly. This means that stable conditions are established very slowly in the backwater cycle. During this adjustment phase, production must often be slowed down to achieve the required paper quality. Consequently, production and quality losses occur.
Another process and device for mixing suspensions is known from the (Published) German Patent Application No. DE 195 09 522 A1, published Sep. 26, 1996. In this document, a stock suspension is fed to a headbox, that is sectioned over the width of the machine, through a plurality of lines feeding stock suspension. The lines feeding stock suspension are connected to a distributor. A portion of the backwater arriving in the drainage region of the paper machine is fed sectionally to the headbox, and is used for basis weight control according to the well-known dilution principle. The remainder of the backwater is passed into a backwater tank, and sent back from the backwater tank into the stock preparation system (xe2x80x9cstable sectionxe2x80x9d), although the stable section is not shown in DE 195 09 522 A1.
In view of the shortcomings of the prior art, an object of the invention is to provide a process for the mixing of suspensions of different nature and/or composition in the stable section of a paper machine, which provides an improvement in the quality as well as a reduction in production loss at the time of the changeover between types. It should be noted that in the context of the specification and claims, suspensions having different xe2x80x9ccharacteristicsxe2x80x9d have differing natures and/or compositions.
A further object of the invention is to provide a mixing device and/or piping in the stable section of a paper or cardboard machine for the blending of suspensions with higher solid content into a first suspension with little or no solid content, which likewise effects a reduction in quality losses and production loss at the time of the changeover between types. It should be noted that in the context of the specification and claims, a xe2x80x9cnegligiblexe2x80x9d solid content means little or no solid content.
According to a first aspect of the present invention, a process for mixing suspensions having differing characteristics in the stable section of a paper machine includes piping of a first suspension in a mixing tube to form a main flow having a main flow direction in a longitudinal direction of the mixing tube, and injecting one or more additional suspensions into the mixing tube. The additional suspension(s) may have a different solid content than the first suspension.
By means of the process according to the invention, larger backwater tanks are avoided, thereby reducing the amount of water in circulation in the paper machine, and, thus, at the time of the type changeover in the paper machine, a more rapid change in the composition of the stock suspensions is possible. Based on this more rapid change, the quality losses, and therefore also the production losses, are reduced. The xe2x80x9cbackwaterxe2x80x9d indicates the total circulating backwater with which, along with the fresh stock, the concentration of the stock suspension required in the headbox is obtained, as depicted in FIG. 1. The cycles in the stable section are described in detail in the literature.
Optionally, a solid content of an additional suspension injected downstream along the main flow direction is, in each case, higher than or equal to a solid content of another additional suspension injected upstream along the main flow direction.
Further optionally, the first suspension includes a suspension of a backwater of the paper machine, and an entire backwater volume stream flows through the mixing tube. In this case, the backwater volume stream may be reduced by a backwater substream sufficient, according to the dilution water principle, for weight basis control on a headbox of the paper machine.
In a particularly advantageous embodiment of the process, flow directions of each of the injected additional suspensions coincide with the main flow direction.
Another embodiment of the process according to the invention includes maintaining a flow rate of the main flow in the mixing tube at a substantially constant level despite added liquid in the injected additional suspension, the flow rate of the main flow in the mixing tube increasing only in an end region of the mixing tube. For a rapid type changeover without losses with respect to paper quality, the residence time of the backwater in the system should be as short as possible. Consequently, the flow rate in the mixing tube is optionally greater than 0.2 m/s, and further optionally, greater than 0.45 m/s (e.g., the dimensions of the mixing device are arranged to maintain these numerical flow rates).
It is also advantageous if each additional suspension is injected concentrically in the main flow. If the recirculation from the headbox is not piped into the mixing tube, recirculation from a headbox may be passed via a line to a vertical separator second stage.
In one particular variation, the first suspension includes a backwater stream of the paper machine, and the injections of additional suspensions include, in order along the main flow, injection of recirculation from a headbox, followed by injection of accepted stock from a vertical separator second stage, followed by injection of recirculation of a first cleaner stage, followed by injection of accepted stock from a second cleaner stage, followed by injection of fresh stock. Although this sequencing of the insertions in the direction of flow is particularly advantageous, additional suspension streams may be injected between, before, or after the recited order, or the sequence may be adapted according to the concentration gradient in view of other conditions present, relative to the concentration of the suspension streams. Moreover, the language xe2x80x9cfollowed byxe2x80x9d is not intended to preclude preceding, intervening, or following process operations after any individual injection, group of injections, or all the injectionsxe2x80x94other process operations may be placed in such positions without departing from the spirit of the invention.
In another particular variation, the first suspension includes a backwater stream of the paper machine, and the injections of the additional suspensions include, in order along the main flow, injection of accepted stock from a vertical separator second stage, followed by injection of recirculation from a first cleaner stage, followed by injection of accepted stock from a second cleaner stage, followed by injection of fresh stock. With this variation, pulsations originating from the headbox and changes in recirculation do not affect the stability of the stable section of the paper machine.
In still another variation, the first suspension includes a backwater stream of the paper machine, and the injections of the additional suspensions include, in order along the main flow, injection of accepted stock from a vertical separator second stage, followed by injection of accepted stock of a second cleaner stage, followed by injection of excess from a stock suspension feed to a headbox, followed by injection of fresh stock.
In yet another variation, the process further includes feeding a first backwater fraction of a backwater stream of the paper machine as the first suspension into a first mixing tube, and feeding a second backwater fraction of a backwater steam of the paper machine as the first suspension into a second mixing tube. In the first mixing tube, the injections of the additional suspensions include, in order along the main flow, injection of accepted stock from a vertical separator second stage, followed by injection of accepted stock of a second cleaner stage, followed by injection of fresh stock. In the second mixing tube, the injections of the additional suspensions include injection of accepted stock from a first cleaner stage.
If there is a steamer on the headbox a return flow from the steamer may be passed via a feed line to a vertical separator second stage.
Another advantageous embodiment of the process provides that the additional suspension(s) is added via a nozzle surrounded by the main flow. The flow rate vD in the nozzle and a flow rate vU of the main flow in a region surrounding the nozzle vU are in a ratio vD/vU from 3 to 15. Maintaining this relationship particularly favors a thorough mixing of the individual liquids.
In still another advantageous embodiment, a region of mixing, i.e., in the region of the addition of the suspensions with higher solid content, between the first suspension and the additional suspension(s) is a hydraulically closed system, preventing equalization of pressure with the surrounding areas. Thus, advantageously, the entire hydraulic system between the paper machine and the stock stream of the headbox can have a closed construction. In other words, there are no free surfaces of the suspension exposed to surrounding areas.
In another variation, a plurality of additional suspensions are injected into the main flow, and volume flow increases downstream along the main flow. The volume flow of the last injection added is smaller than the volume flow of the next to last injection added. Alternatively, the volume flow of the next to last injection added is greater than the volume flow of the last injection added.
In a modification, the injection of the additional suspension(s) includes wherein the injection of additional suspension(s) includes injection of a plurality of ingredients of fresh stock via a plurality of corresponding feeds in substantially the same location along the main flow.
According to another aspect of the present invention, a mixing device for the blending of additional suspensions into a first suspension in the stable section of a paper machine includes a tube, and an intake in the tube for the first suspension, the first suspension having a negligible solid content. A plurality of feeds into the tube are provided for the additional suspensions to be blended with the first suspension into a blended suspension with a new solid content, the additional suspensions having higher solid content than the first suspension. An outlet in the tube is provided for the blended suspension, the outlet being disposed downstream from a bend in the tube. A pump is connected to the tube downstream from the outlet, wherein an impeller axis of the pump is perpendicular to a plane containing portions of the tube both upstream and downstream of the bend. That is, the mixing device is preferably arranged perpendicularly and has at its lower end a bend with a connection to the downstream pump (e.g., the cleaner pump). The plane of the bend and the perpendicular part of the mixing tube is perpendicular to the axis of rotation of the downstream pump. This ensures uniform inflow, in particular with double-suction pumps. It should be noted that a xe2x80x9cmixing devicexe2x80x9d can include a mixing device and associated piping.
Advantageously, each of the plurality of feeds includes an injection site that injects an additional suspension having a solid content equal to or greater than a previous injection site of a previous feed along the downstream direction of the main flow. In other words, the concentration or the solid content of the suspensions added should increase continuously or remain the same in the direction of flow. The concentration differences at the individual mixing points are minimized, which ensures high mixing efficiency and low fluctuations in concentration. In this case, each of the injection sites may include an outlet port with each outlet port pointing in a direction of the main flow. In this manner, the flow directions of the main flow and the added suspension(s) have essentially the same orientation.
Optionally, an internal diameter of the mixing device is designed such that a flow rate of the main flow is maintained at a substantially constant level despite added liquid in the additional suspensions blended therein, and such that the flow rate of the main flow in the mixing device increases only in an end region of the mixing device.
Each of the plurality of feeds may include an injection site, and each injection site may terminate centrally in the mixing device.
If the recirculation from the distributor of the headbox is not passed into the mixing device, a recirculation line from a distributor of a headbox may pass via a line to a vertical separator second stage. Moreover, a return flow line from a steamer of a headbox may be passed via a line to a vertical separator second stage.
In one particular variation, the feeds into the tube for the additional suspensions include, in order along the main flow, a feed for recirculation from a headbox, followed by a feed for accepted stock from a vertical separator second stage, followed by a feed for recirculation of a first cleaner stage, followed by a feed for accepted stock from a second cleaner stage, followed by a feed for fresh stock. As noted above, although is sequencing of the feeds in the direction of flow is particularly advantageous, additional feeds may be provided between, before, or after the recited order, or the sequence may be adapted according to the concentration gradient in view of other conditions present, relative to the concentration of the suspension streams. Moreover, the language xe2x80x9cfollowed byxe2x80x9d is not intended to preclude preceding, intervening, or following structure after any individual feed, group of feeds, or all the feedsxe2x80x94other structure may be placed in such positions without departing from the spirit of the invention.
In another particular variation, the feeds into the tube for the additional suspensions include, in order along the main flow, a feed for accepted stock from a vertical separator second stage, followed by a feed for recirculation from a first cleaner stage, followed by a feed for accepted stock from a second cleaner stage, followed by a feed for fresh stock. In this manner, addition of the recirculation from the headbox is eliminated, avoiding possible pressure fluctuations and pulsations in the stable section which could be transferred by the recirculation from the headbox.
In still another particular variation, the feeds into the tube for the additional suspensions include, in order along the main flow, a feed for accepted stock from a vertical separator second stage, followed by a feed for accepted stock of a second cleaner stage, followed by a feed for excess from a stock suspension feed to a headbox, followed by a feed for fresh stock.
In yet another particular variation, the tube includes a first mixing tube provided for a first backwater fraction of a backwater stream of the paper machine, and a second mixing tube for a second backwater fraction of a backwater stream of the paper machine. The feeds into the first mixing tube for the additional suspensions include, in order along the main flow, a feed for accepted stock from a vertical separator second stage, followed by a feed for accepted stock of a second cleaner stage, followed by a feed for fresh stock. The feeds into the second mixing tube for the additional suspensions include a feed for accepted stock from a first cleaner stage.
Optionally, each of the plurality of feeds includes an injection site surrounded by the main flow, and inside diameters of the injection sites and an inside diameter of the mixing device in the region of the injection site are arranged such that a flow rate vD in each injection site and a flow rate vU of the main flow in a region surrounding the injection site are in a ratio vD/vU from 3 to 15. As noted above, maintaining this relationship particularly favors a thorough mixing of the individual liquids.
Further optionally, the mixing device is a hydraulically closed system excepting the intake and the outlet port for the blended suspension. That is, the mixing device is closed relative to its surroundings, or constitutes a closed hydraulic system having no pressure equalization capability with its surroundings. As noted above, the entire hydraulic system between the paper machine and the stock stream of the headbox can have a closed construction, and there are no free surfaces of the suspension exposed to surrounding areas.
According to still another aspect of the present invention, a mixing device for the blending of additional suspensions into a first suspension in the stable section of a paper machine includes a tube, and an intake in the tube for the first suspension, the first suspension having a negligible solid content. A plurality of feeds into the tube are provided for the additional suspensions to be blended with the first suspension into a blended suspension with a new solid content, the additional suspensions having higher solid content than the first suspension. An outlet in the tube is provided for the blended suspension, the outlet being disposed downstream from a bend in the tube. Each feed of the plurality of feeds injects an additional suspension having a solid content equal to or greater than a previous feed along the dowstream direction of the main flow.
According to yet another aspect of the invention, a process for mixing suspensions having differing solid content in the stable section of a paper machine, includes feeding a backwater suspension from a wet section of the paper machine as a main flow into a closed vertical mixing tube, then injecting accepted stock from a vertical separator system concentrically into the main flow to form a blended suspension in the mixing tube, the injection of the accepted stock having a higher solid content than the backwater suspension and a higher flow rate than the main flow. Fresh stock is then injected concentrically into the blended suspension in the mixing tube, the injection of the fresh stock having a higher solid content than the blended suspension and a higher flow rate than the blended suspension, then the blended suspension is pumped from the mixing tube.
In this case, a flow rate in the mixing tube may be maintained at a substantially constant level upstream and downstream of the injections.
According to still yet another aspect of the invention, a mixing device for the blending of additional suspensions into a backwater suspension in the stable section of a paper machine includes a closed vertical mixing tube having a bend at a lower end thereof. An intake is provided at a top of the tube for a backwater suspension from a wet section of the paper machine, the backwater suspension forming a main flow. A first concentric nozzle injects accepted stock, having a higher solid content than the backwater suspension, from a vertical separator system into the mixing tube to form a blended suspension. The first concentric nozzle is concentric to the mixing tube and upstream of the bend, and injects the accepted stock at a higher flow rate than the main flow. A second concentric nozzle injects fresh stock having a higher solid content than the blended suspension into the mixing tube. The second concentric nozzle is concentric to the mixing tube and downstream of the bend, and injects the fresh stock at a higher flow rate than the blended suspension. An outlet in the tube is disposed downstream from the bend and from the second concentric nozzle.
In his case, a diameter of the mixing tube may increase in the direction of the main flow to maintain a flow rate in the mixing tube at a substantially constant level upstream and downstream of both of the first concentric nozzle and the second concentric nozzle.
Accordingly, with the invention as described, it is possible to omit the expensive backwater tanks and, if necessary, to form a closed hydraulic system. This results in shorter residence times of the suspension return flow and a more rapid stabilization of the hydraulic system with regard to concentration and suspension composition after a type changeover. Thus, reduced production loss and fewer quality losses are achieved.